Reactions of Alkenes II

1
Chapter 9

• Reactions of Alkenes (II)

2
Hydrogenation of alkenes

• The addition of H2 to a double bond leads to the
  saturated alkane
• The most commonly used catalyst is Pd on carbon
  (Pd/C)

3
Addition of H2 occurs syn

• Both hydrogens (or deuteriums in this case) end
  up at the same face of the molecule.
• There is a 100 selectivity without exceptions

4
The supposed mechanism

• Adsorption leads to more reactive bonds of both
  the alkene and the hydrogen gas.
• This is the reason of the syn mode of addition

5
Rate of hydrogenation

• The more stable the double bond, the slower the
  hydrogenation
• In other words the more substituted, the slower
  the hydrogenation of the double bond proceeds

6
Steric effects

• The hydrogen usually reacts at the least hindered
  site of the molecule

7
Addition of halogens

• Reaction of alkenes with molecular bromine (Br2)
  and chlorine (Cl2) give vicinal dihalides
• Vicinal means on adjacent (1,2) positions
• The reaction gives selectively anti addition

8
Two possible mechanisms

• The reaction proceeds exclusively via the
  so-called bromonium ion this explans the
  anti-product
• Is the product optically active?

9
Competition with other nucleophiles

• In the presence of different nucleophiles,
  different products are formed, depending on their
  nucleophilicity and concentration

10
Question

• How many products do you expect for the following
  reaction? Explain the answer.

11
Several possibilities

• The ratio depends on the relative concentrations
  of the nucleophiles

12
100 anti addition of H2O

• Bromohydrin formation the mode of addition is
  similar (anti) to the reaction with Br2 itself

13
Question

• Predict the major product of the following
  reaction and explain the answer.

14
Problems

• Make problems 9.33 a-e, 9.35

15
Addition of H2O revisited

• Oxymercuration of an alkene with Hg(OAc)2,
  followed by reduction with NaBH4 leads to
  overall addition of H2O

16
The mechanism

• The mechanism proceeds via a three-membered ring
  mercurinium ion
• The stable three membered ring prevents
  carbocation rearrangements

17
The reduction

• NaBH4 is a hydride (H) donor the hydride
  replaces the mercury atom

18
Other oygen donors

• Most peroxides and peracids are potential
  explosives!!
• This is because they easily loose an oxygen atom

19
Peracids react with double bonds
mCPBA

• Peracids react with alkenes to form epoxides
  (oxiranes)
• The most used peracid is m-chloroperbenzoic acid
  (mCPBA)

20
The stereochemistry is retained

• (E)-alkenes lead to trans-substituted epoxides
  and (Z)-alkenes lead to cis-substituted epoxides
• The double bond geometry is conserved in the
  product

21
Ring opening of epoxides

• Epoxides can be opened under basic and acidic
  conditions

22
Question

• Predict the outcome of the following reactions.
  Explain the answer.

23
Two different mechanisms

• Why is the oxygen anion (usually a poor leaving
  group) in this case a good leaving group?

24
Other nucleophiles

• As long as there are no cations involved, other
  nucleophiles (such as the azide) will open the
  epoxide from the least hindered side

25
Addition of HX

• Opening with strong acids proceed via the
  cationic intermediate

26
Problems

• Make problems 9.31 a-f, 9.37 (de uitgangsstof
  moet cis-2-buteen zijn!)

27
Figure 9.40
Carbenes

• Carbenes can be generated in two ways

• Carbenes are species containing a carbon atom,
  that bears a free electron pair (and only six
  electrons)

28
Carbenes and alkenes give cyclopropanes

• The carbene lone pair reacts with alkenes to
  cyclopropanes
• The geometry of the double bond is preserved in
  the product the addition is syn

29
The mode of addition
HOMO-LUMO interactions change through the
addition process
30
1,3-Dipolar compounds

• Examples of reagents that contain a 1,3-dipole a
  positive and a negative charge in the same
  molecule

31
A well-known 1,3-dipole is O3

• Ozone reacts very fast (at 78 ºC) with alkenes
  to form a five-membered ring (the primary
  ozonide)
• This is a syn addition the double bond geometry
  is retained

32
The fate of the primary ozonide

• The primary ozonide rearranges almost immediately
  to the more stable ozonide
• This proceeds via a reverse 1,3-dipolar addition

33
Figure 9.65
The fate of the ozonide (I)

• Tekst

• Reductive workup leads to aldehydes and ketones
• Reductive workup Me2S, or PPh3, or Zn

34
Figure 9.66
The fate of the ozonide (II)
Oxidative workup (rxn with H2O2) leads to ketones
and carboxylic acids.
35
Problems

• Make problem 9.32 e,f, 9.41 and 9.42

36
Other oxidation rxns of alkenes

• Alkenes react rapidly with the permanganate ion
  (MnO4), but also with OsO4 (osmium tetroxide)

37
Hydrolysis leads to diols

• Propose a reasonable mechanism for the hydrolysis
  of the cyclic osmate ester

38
Examples

• In all cases, the mode of addition is syn

39
Problems

• Make problem 9.38

40
Addition reactions to alkynes

• Alkynes behave very similar to alkenes in all
  previous reactions

41
Addition of HX to alkynes

• The trend in stability of vinyl cations is
  similar to the normal cations
• Vinal cations are significantly less stable than
  saturated carbocations
• What is the hybridization of a vinyl cation?

42
A terminal alkyne

• The chloride adds to the more stable secondary
  vinyl cation Markovnikov addition

43
Other examples

• Explain the outcome of the reaction with
  2-chloropropene

44
Reaction with Cl2

• Draw a plausible intermediate for these reactions

45
Oxymercuration gives ketones

• The oxymercuration follows the same pathway as
  with alkenes in the presence of acid, the
  product breaks down to give an enol, which
  tautomerizes to the ketone

46
The keto-enol tautomerization

• Note tautomers are not mesomeric structures,
  they are different species, which are (in
  principle) in equilibrium with each other

47
Hydroboration of alkynes

• Note an (E)-substituted double bond is formed,
  resulting from syn-addition

48
Hydrolysis leads to aldehydes

• Hydrolysis leads to an enol, which again
  tautomerizes to a carbonyl in this case the
  anti-Markovnikov mode of addition leads to an
  aldehyde.

49
Hydrogenation of alkynes (I)

• In the presence of Pd/C and H2-gas, alkanes are
  formed

50
Formation of (Z)-alkenes

• Hydrogenation in the presence of the Lindlar
  catalyst (a poisoned Pd-catalyst), the
  hydrogenation stops in the alkene stage
• An (Z)-alkene is formed resulting from
  syn-addition of H2

51
Formation of (E)-alkenes

• In contrast, reduction of the acetylene with Na
  in NH3 leads to the (E)-alkene

52
The mechanism
53
The mechanism (II)

• Na in NH3 is a very strong reductive medium
• It can donate electrons to the triple bond in a
  stepwise manner
• The radical intermediate equilibrates to the most
  stable isomer

54
Problems

• Make problems 9.30, 9.51